It is widely known that variable valve actuation can be used to improve fuel economy and emissions by reintroducing hot exhaust gasses into the combustion chamber. This is often referred to as internal exhaust gas recirculation (EGR). It is distinguished from external EGR which is effected by way of a passage that leads from the exhaust to the intake manifold and includes an EGR valve.
One method of generating internal EGR involves the re-opening of the exhaust valve during the induction stroke. As the piston moves down the cylinder, exhaust as well as intake gases are introduced into the cylinder prior to compression and ignition.
EP 1649148 shows an example of a continuously variable lift system, which sums the lift from two separate cam profiles, to reopen the exhaust valve during the induction stroke. This system allows for precise control over the amount of internal EGR generated as the secondary exhaust valve lift is continuously variable.